A router in a backup path in an MPLS network adopting a label stack method aims to achieve the same quality-guarantee control as the one performed in a current path. In a packet transfer device accommodating first and second output lines, when the first line has a fault, upon a first packet being received, communication quality information in a header of the first packet is copied. A first header containing the copied priority information and a new label is added to the first packet, and the first packet is outputted to a second output line. When a second packet is received, communication quality information in a header of the second packet is copied, a second header containing the copied priority information and the new label is added to the second packet, and the second packet is outputted to the second output line.